Secondary Hemostasis

Substrate

The main reactant is fibrinogen

Cofactors in Secondary Hemostasis

Proteins that accelerate enzymatic reactions

• Factor V

• Factor VIII

Enzymes in Secondary Hemostasis

Two types

• Transaminase that cross-linked the fibrin: factor XIII

• Serine proteases that cleave peptide bond: The other factors

Contact Group Factors {Properties}

Contact activation and initiate intrinsic pathway

• Factor XII, prekallikrein (PK), high molecular weight kininogen (HMWK)

• Not consumed during coagulation

• Not vitamin-K dependent

• Present in serum

Prothrombin Group Factors {Properties}

Generating & amplifying coagulation

• Factor II, factor VII, factor IX, factor X

• Not consumed (except factor II)

• Vitamin-K dependent

• Present in serum

• Adsorbed by BaSO4

Thrombin-Sensitive Group {Properties}

Involved in fibrin formation & clot stabilization

• Factor I, Factor V, Factor VIII, Factor XIII

• Consumed during coagulation

• Not vitamin-K dependent

• Absent/markedly reduced in serum

Labile Factors

Easily inactivated

• Factor V

• Factor VIII

Intrinsic Factors

• Factors XII, XI, IX, VIII

• Test by aPTT

Extrinsic Factors

• Tissue thromboplastin (TF III), Factor VII

• Test by PT

Common Pathway

• Factors X, V, II, I, XIII

• Test by both aPTT and PT

Intrinsic Tenase Complex

• IXa + VIIIa + Ca²⁺ + phospholipid surface

• Cleave X → Xa

Prothrombinase Complex

• Xa + V + Ca²⁺ + phospholipid surface

• Cleave prothrombin (II) → thrombin (IIa)

Extrinsic Tenase Complex

• TF III + VIIa + Ca²⁺

• Cleave X → Xa

Thrombin Functions

• Forms fibrin

• Activates platelets

• Amplifies the cascade by activating factor V, factor VIII, XI

• Stabilizes clot by activating factor XIII

Fibrin Polymerization

• fibrin monomer combines with fibrinopeptides A & B to form weak fibrin polymer

• Factor XIIIa and calcium cross-link fibrin polymer via covalent bond to form stable fibrin clot